A typical electronic image sensor comprises a number of light sensitive picture elements (“pixels”) arranged in a two-dimensional array. Such an image sensor may be configured to produce a color image by forming an appropriate color filter array (CFA) over the pixels. Examples of image sensors of this type are disclosed in U.S. Patent Application Publication No. 2007/0024931, entitled “Image Sensor with Improved Light Sensitivity,” which is incorporated by reference herein.
As is well known, an image sensor may be implemented using complementary metal-oxide-semiconductor (CMOS) circuitry. In such an arrangement, each pixel typically comprises a photodiode and other circuitry elements that are formed in a silicon sensor layer on a silicon substrate. One or more dielectric layers are usually formed above the silicon sensor layer and may incorporate additional circuitry elements as well as multiple levels of metallization used to form interconnects. The side of the image sensor on which the dielectric layers and associated levels of metallization are formed is commonly referred to as the frontside, while the side having the silicon substrate is referred to as the backside.
An image sensor formed in multiple layers as described above may be viewed as an example of an arrangement commonly referred to as a stacked image sensor. Such a stacked image sensor may be formed from a single semiconductor wafer. Other types of stacked image sensors may be formed from separate sensor and circuit wafers that are arranged in a stack and interconnected with one another.
Image sensors may be generally classified as either frontside illuminated or backside illuminated. In a frontside illuminated image sensor, light from a subject scene is incident on the frontside of the image sensor, and the silicon substrate is relatively thick. However, the presence of metallization level interconnects and various other features associated with the dielectric layers on the frontside of the image sensor can adversely impact the fill factor and quantum efficiency of the image sensor.
A backside illuminated image sensor addresses the fill factor and quantum efficiency issues associated with the frontside dielectric layers by thinning or removing the thick silicon substrate and arranging the image sensor such that light from a subject scene is incident on the backside of the image sensor. Thus, the incident light is no longer impacted by metallization level interconnects and other features of the dielectric layers, and fill factor and quantum efficiency are improved.
However, similar improvements in fill factor and quantum efficiency have been difficult to achieve in frontside illuminated image sensors. This is in part due to the height of the image sensor stack, which tends to limit reductions in pixel size as well as improvements in fill factor. Also, when using conventional techniques for forming frontside illuminated image sensors, it can be difficult to precisely control the characteristics of the photodiode depletion regions so as to ensure sufficient charge carriers. Failure to configure the photodiode depletion regions to provide sufficient charge carriers can degrade quantum efficiency and resulting image quality.
Accordingly, a need exists for improved techniques for forming image sensors, which can achieve reduced stack height, smaller pixel sizes and higher fill factor than the conventional techniques without adversely impacting quantum efficiency or image quality.